Limusaurus confusion

The advent of the description of Limusaurusand the associated hypothesis of digit homologies in the paper is likely to generate quite a lot of interest in dinosaurian and avian circles. However, it will, I suspect, also generate a fair bit of confusion in the short and perhaps even long term, even if the hypothesis is rapidly refuted or is not adopted by the palaeontological community. It’s a problem that comes up occasionally in palaeontology (and I imagine other fields too) and is worth commenting on at least a little.

The issue is that there are (for now) two working hypotheses for the homologies of digits in most theropods, either as I-II-III, or as II-III-IV. Until one or the other wins out (or even if one does not) we have to be very careful with how we refer to theropod digits. Obviously Limusaurus has generated much discussion in the office and by e-mail during the writing of the paper, and we were forced to resort to referring to ‘traditional’ and ‘new’ interpretations of patterns (or even single digits) to avoid confusion and remove ambiguity. Others are going to have to follow this pattern in their descriptions and analyses or an author may be referring to a digit II in the new sense and a reader take it as a digit II in the traditional sense and the two are very different.

In the short term this issue will probably also fuel a number of new investigations, revisions of old ones, or duplicates to see how the two different hypotheses affect the results of their work. Some initial work in the paper suggests that the II-III-IV hypothesis is more parsimonious in cladistic analyses by reducing the number of transitions in theropods, and while this is only tantalising evidence that the new hypothesis may be correct, it does illustrate the importance of investigating both hypotheses in parallel.

In the long term the presence of two hypotheses or the rejection of the traditional one (if that happens) may become really confusing. We have more than 150 years of research papers describing theropod digits as I-II-III and this may change. It does not make them obsolete, and of course it is relatively easy to mentally flick the switch when reading in order to change I-III as II-IV but it is complex. More importantly, when a new student or non-specialist gets to the literature (let’s say in 20 years or so), unless he meets the debate of the hypotheses early on, he will really struggle to work out how and why the transition occurred from one to the other and which papers are right and how the digits should be interpreted. I was stuck in a realm of confusion over a very similar issue during my PhD studies where I missed the fact that there were (and indeed still *are*) two different definitions of the archosaurs. I was reading papers that would each be using a different definition but were not always explicit about the differences or which definition they were using and it was quite a time before I found out and was able to put it into context and sort it out in my own mind. I can imagine something similar happening here in the future (depending on how it all works out, obviously).

Right, that leaves me pretty Limusaurus-ed out for now. No doubt the debate will rage, perhaps on here, but also elsewhere over exactly what this is and what it means. It will take time, but regardless of the eventual result there is some fascinating research to come.

I believe it was Jim Clark (thought I don’t actually know for certain). Actually it was pretty popular wih the authors though that may be becuase the original chosen was terrible. And while we are on the subject, don’t forget that there were two animals trapped in the mud, not one as you sugested in the Tet Zoo comments (thiught I agree at least that other options were available, we did at least dodge another bleedin’ ‘raptor’ name).

“More importantly, when a new student or non-specialist gets to the literature (let’s say in 20 years or so), unless he meets the debate of the hypotheses early on, he will really struggle to work out how and why the transition occurred from one to the other and which papers are right and how the digits should be interpreted.”

Well, it’s tricky and complicated with lots of caveats / alternates, yes, and I was referring to the main paper (which of course does not go into all the details ofn the extra analyses) and not the supplementary info (and I did say ‘suggests’) to try and keep things clear. I’ll try and get the author of that section to comment on here about the work he did.

And of course, Limusaurs could just demonstrate an extreme form of arm/hand reduction present in other ceratosaurs, and thus not tell us a whole lot about frame shifting at all. This is not to say the frame shift did not occur at some juncture (it must have!), but I’m not convinced that Limusaurus represents that frame shift.

But that’s part of the point Zach – even if Limusaurus does not represent the condition we hypothesis, this does not mean the hypothesis itself is not valid (thought it would reduce support for it, certainly). This still provides us with a new hypothesis for the apparent switch from I-III to II-IV and is in itself interesting (I would hope) and requires study.

I was wondering if Limusaurus is the same as the small Shishugou taxon tentatively identified by Clark et al. (2002) as a basal ornithomimosaur? If so, what of the two additional specimens Clark et al. mentioned?

Yes, but I’m not sure people would necessarily realise this, hence my comment. A lot of general readers do read this stuff and not everyone knows that the hypothesis and the evidence are not necessarily fundamentally intertwined.

For a start, every theropod publication for the next several decades is going to “have to have” a footnote explaining which hypothesis/convention is in force before using Roman numerals for digits, even in purely descriptive papers! This is a serious potential cause of confusion; I’d suggest that the “have to have” an explanatory footnote ought to be enforced by referees.

I’m not sure that’s the case, like I say, our one may quickly die a horrid death. Certainly I’d agree if oth reign at least for a while people need to be more specific about how they are describing / defining things and this has not always been clear in the past (as with the definition fo archosaurs for example).

Here is our response to the Limusaurus paper that was recently rejected by nature, not for any technical reason but because it was considered not to be of sufficient interest/importance.

We have uploaded it at the nature precedings citable archive, because we think it is important there is a quick and citable reply that unlike Xu’s proposal, is consistent with the view of the larger community of theropod paleontologists, namely, that tetanuran digits still are I, II, III. We are preparing a longer paper on this topic.

As A. Vargas must be aware, we had a reply ready for Nature for their reply, but with the latter being rejected so too was ours which of course is now sitting unused on a hard-drive. I’ll not comment more here since while I’m in the UK I don’t have access to this paper and since I suspect the team would like to prepare a formal reply I should not comment here in any case. Suffice to say there is more to come on this, both from the original Limusaurus team and I suspect many others.

Well I’m still on holiday in the UK and my co-authors are in the US and China so I’m not really in a position to do much about it right now, and obviously despite my involvement, this is not ‘my’ project to make any decisions on. I’m sure we will do something with it sooner rather than later.

I was really just trying to get the point across to readers that this kind of scientific dialogue is important and ongoing. I actually try to keep the research off these pages until it’s published and just let people know that there was likely more to come.

We did, but then Nature Pre. took it down because there was a problem with the link and then never put it back up. (and it was Xu et al. rather then Choiniere et al.).

Since Xu is working on a longer paper about the issues (which i may or may not be involved in) and since a formal reply from Vargas has not actually appeared I don’t think anyone was too put out by this brief appearance and then disappearance. I mean, to a certain degree only the literature counts as such. We could trade endless unreviewed replies in NP, or online or wherever but it’s not a huge amount of use really. Formal papers are needed and neither side has got that far (obviously beyond the original description of Limusaurus).